New research from the Royal Veterinary College has revealed that a chromosomal defect is the reason why a significant proportion of horse pregnancies fail within the first two months of development.

The College says this will pave the way for new diagnostic tests for what could be one of the most common causes of pregnancy loss in mares.

The researchers, led by Dr Mandi de Mestre (pictured right), Reader in Reproductive Immunology and Head of the Equine Pregnancy Laboratory at the RVC, collaborated with seven different veterinary practices to gain access to samples from across the UK and Ireland and found that around 20% of the pregnancies lost were aneuploid, which is when a copy of a whole chromosome is either duplicated or lost (similar to Down's).

Charlotte Shilton, RVC PhD, the student who performed the analysis, applied three different genetic approaches to confirm the results.

Work is now underway to identify the underlying cause of these aneuploid pregnancies, with early data from this study suggesting it is most commonly introduced via the egg or sperm. Until now, chromosomal defects such as aneuploidy have only been reported as a rare condition in young horses with developmental disorders. 

The researchers say the study explains why the condition is so rare in horses, with most embryos and foetuses possessing this genetic change dying very early in development, as is also observed in human pregnancy. The study highlights the need to reconsider this genetic condition both in pregnancy loss but also for early developmental disorders.

Dr Mandi de Mestre, Reader in Reproductive Immunology at the RVC, said: “Early pregnancy loss remains a very frustrating condition for clinicians to treat as the underlying cause is unknown in around 80% of cases. These findings will allow researchers to develop new diagnostic tests for pregnancy losses, which would offer hope to thousands of owners of breeding mares that suffer this condition.

“A diagnostic test would allow them to make informed decisions on treatment strategies and to advise on whether they should invest in further attempts to breed their mare benefiting both horses and their breeders alike in the future. I would like to thank both the Thoroughbred Breeders Association and our collaborators at Texas A&M University and the participating veterinary surgeons for their support on this project.

Reference

1. 10.1038/s41598-020-69967-z "Whole genome analysis reveals aneuploidies in early pregnancy loss in the horse" www.nature.com/articles/s41598-020-69967-z

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